1. Field of the Invention
This invention relates to a method of forming a protective coating of castable refractory material on a lance for desulfurizing molten iron, a lance for controlling the temperature of molten steel and other long lances through which a gas or powder is injected into molten metal in which they are immersed. More particularly, the invention relates to a method of forming a protecting coating on a long lance of the above described type, preferably having a length greater than 1 m (such lance is hereunder referred to as a long immersible lance).
2. Description of the Prior Art
A lance having a protective refractory coating is used when molten iron is freed of sulfur or other impurities in a container such as a ladle or torpedo car. But, as accompanying FIG. 1 shows, a protected lance 15 in molten metal 16 is subject to a great degree of erosion at the lower middle portion, in particular, the portion where the lance contacts slag 17 floating on the surface of the metal. A lance which is eroded in the lower middle portion is disconnected for replacement by a new gas injection lance. Therefore, the life of a refractory-coated lance generally depends on the life of the lower middle portion of the protective refractory coating.
FIG. 2(a) shows how an immersible lance conventionally is provided with a protective coating. Within a mold 1 that conforms to the vertical configuration (inverted with respect to the lance position in use mode), a lance 4 is set to provide a clearance that is equal to the coating thickness, and the gap is filled with a coating material 7 that is supplied from a mixer 5 through an opening 2 in the top of the mold 1. FIG. 2(b) is a transversal cross section FIG. 2(a) taken along the line A--A, and it shows one embodiment of the lance 4 set in the mold 1. For uniform packing of the coating material 7 around the lance 4, a vibrator 3 is used. The vertically installed mold is commonly employed because the coating material can be supplied overhead and so, a simple construction of the mold will do.
However, it has been found that if the conventional method is used to form a protective coating on a long lance, it is impossible to form a uniformly dense coating for the length of the lance because for one thing, the upper portion of the lance in the mold is unavoidably covered with a less dense coating than the lower. Also a settable substance such as the aggregate or fibrous material, when attempting to add fibrous material as a reinforcing material, in the coating material settles in the lower portion of the mold. As a result, the lower middle portion of the coating on the lance in the use mode that contacts slag does not have an adequate amount of a reinforcing material (e.g. aggregate or fibrous material) and coupled with the low density described above, that portion does not have as high strength or resistance to slag erosion as does the upper portion of the coating. Consequently, the good properties of the coating material are not reflected in the actual durability of the resultant coating. As shown in FIG. 2(a), the lance 4 is set in the mold 1 in a position inverted to that of the lance in the use mode, so the upper portion of the lance during the coating application corresponds to the lower portion of the lance in the use mode. This means that the portion of the lance which is less strong and erosion-resistant is subjected to severer, hence more unfavorable conditions during use.
Therefore, this invention provides an improved method of forming a protective coating on a long immersible lance.
Several methods have been devised and reported to prevent damage of a castable refractory coating on a long lance of the type contemplated by this invention. One example is described in Japanese Utility Model Publication No. 29286/79 (Japanese Utility Model Application No. 15097/76 "Lance for injecting gas for treatment of molten metal"). It discloses a lance which is surrounded at suitable intervals by a plurality of short steel tubes which are covered with a castable refractory material to prevent the formation of cracks in the castable refractory layer due to the difference in thermal expansion coefficient between the lance and the surrounding castable refractory coating. But the reference has no idea of the technique to improve the erosion resistance of the coating layer, so unlike this invention, it makes no mention at all of the position in which the mold is filled with a castable refractory, although FIG. 1 and the related description in the specification suggest that a vertical position is intended.
The Japanese Industrial Standards (JIS) A5302 describe a method of producing a reinforced concrete pipe. According to this method, concrete is cast into a mold which is erected upright and the concrete is set by vibratory compacting. Therefore, the method differs in construction from this invention which achieves formation of a coating having uniform distribution of grain size and ingredients throughout the length of the coating by pouring a castable refractory into a mold lying on its side through an inlet port provided in the top of the mold, and the refractory is set without erecting the mold upright.